Problem: $-\dfrac{1}{10} + \dfrac{1}{8} = {?}$
Explanation: $ = - {\dfrac{1 \times 4}{10 \times 4}} + {\dfrac{1 \times 5}{8 \times 5}} $ $ = - {\dfrac{4}{40}} + {\dfrac{5}{40}} $ $ = - \dfrac{{4} + {5}}{40} $ $ = \dfrac{1}{40}$